The invention concerns fuel cells, each having a negative pole flange, a negative electrode, a membrane, a positive electrode, and a positive pole flange with at least four lead-through openings for feed and discharge, as well as batteries made of such fuel cells.
Hydrogen/oxygen and hydrogen/air fuel cells deliver a voltage of approximately 1 V. Since this voltage is far too low for practical applications, a plurality of individual cells must be connected electrically in series to obtain the voltage required, for example, for operating motors or electronic systems. These sets of electrically connected individual cells are referred to as "fuel cell stacks."
It is known and customary to manufacture such stacks by repeatedly stacking the individual components of fuel cells and mechanically clamping them together using threaded bolts or other fasteners. Thus, for example, it is also known that the required forces can be produced hydraulically or pneumatically, as is done according to what is usually referred to as "filter press technology" (see e.g., W. Vielstich "Brennstoffelemente" [Fuel Elements], Verlag Chemie GmbH, 1965, pp. 171 and 201-202). By mechanically compressing the stack, all the edges of the individual fuel cells, as well as the required feed-throughs for the operating gases and for any coolant, are sealed. The following pairs of media must be reliably separated using gaskets: hydrogen/oxygen (air), hydrogen/atmosphere, oxygen/atmosphere, and electrolyte/atmosphere, as well as hydrogen/coolant and oxygen/coolant, when a coolant is used. When a liquid coolant is used, it must also be hermetically sealed against the atmosphere. On the other hand, when using air as a coolant, it is desirable to have the least possible hindered exchange with the atmosphere.
The basic components of a fuel cell are--in the order of assembly--the following (see FIG. 1): an electronically conductive contact plate (11) forming the negative pole of the individual cell (10) with devices (channels or bores) for distributing hydrogen (12), a porous hydrogen electrode (13), a porous electrolyte carrier (14), a porous oxygen or air electrode (15), and an electronically conductive contact plate (16) forming the positive pole of the individual cell with devices (channels or bores) for distributing oxygen or air (17). The porous electrolyte carrier (14) is soaked with liquid electrolyte before or after assembly. An ion exchange membrane can also be used as a porous electrolyte carrier in particular. Fuel cells equipped with such membranes, called PEM (Proton Exchange Membrane or Polymer Electrolyte Membrane) fuel cells, are considered especially promising energy sources for automobile engines (see: VDI Report No. 912 (1992), pp. 125-145).
In the known fuel cell stack designs, there are individual contact plates (11 and 16) only at the positive and negative ends of the stack. The contact plates inside the stack are combined to form "bipolar plates," which often have a hollow design. A coolant may flow through such contact plates, in which case they are referred to as cooling plates. Bipolar plates are therefore a typical element of the known fuel cell stack designs.
Among the previously mentioned sealing points the most critical by far is the separation of hydrogen from oxygen. If this seal is faulty, hydrogen may come in contact with the oxygen or air electrode, or oxygen (air) may come in contact with the hydrogen electrode. In either case, combustible gas mixtures are formed, which are likely to be catalytically ignited by the electrode materials, which may result in the destruction of the entire stack. Therefore, for safety reasons, the cells and stacks must be designed so that direct hydrogen/oxygen (air) seals are avoided. In a cell that is properly designed in this respect, hydrogen and oxygen escape to the atmosphere if a seal is defective. If there are no sources of ignition there, no danger is present. Although this can be achieved using filter press technology for edge sealing by having the membranes extend into the atmosphere, there is the risk of the membranes drying out at the edges, which may result in considerable corrosion problems and--after extended shutdown periods--also to operating problems. These sealing problems normally occur not only at the cell edges, but also at the cell-to-cell feed-throughs.
An important disadvantage of filter press technology consists of the fact that the dimensional tolerance requirements for the edge seals are very strict or only highly elastic materials can be used, which require complicated processing and also have numerous other disadvantages. Furthermore, this technology has the disadvantage that all sealing surfaces are produced simultaneously when the stack is assembled. When sealing defects occur, they can only be located using special procedures. In addition, fittings of only relatively small cross sections can be used with filter press technology to supply the cells with reactants and coolant. In particular, the wide flow channels required for air cooling are almost impossible to implement. On the other hand, when liquid coolant is used, electrochemical corrosion at the required high operating voltages, e.g., 200 V, is difficult or impossible to control.